1. Field of the Invention
The present invention relates to a nickel/metal hydride storage battery which uses an improved nickel active material for positive electrode having an increased energy density and possesses improved characteristics of the positive electrode and hence improved characteristics of the battery. In detail, in the present invention, nickel hydroxide as the active material of the positive electrode is essentially subjected to reduction and oxidation, i.e., discharge and charge reactions, while essentially assuming .beta.-phase. In further detail, the discharge and charge reactions of nickel hydroxide are carried out at an average valence (average oxidation valence) of nickel of less than 3.5 and not less than 2.1.
2. Description of the Related Art
In recent years, alkaline storage batteries have been desired to be increased in capacity, with the spread of portable electric devices. In particular, nickel/metal hydride storage batteries are secondary batteries comprising a positive electrode made of an active material composed mainly of nickel hydroxide and a negative electrode composed mainly of a hydrogen-absorbing alloy, and are being rapidly spread as secondary batteries having high capacity and reliability.
Positive electrodes for alkaline storage batteries are roughly classified into sintered type and non-sintered type. The sintered type positive electrodes are produced by preparing a porous nickel sintered plaque with a porosity of about 80% by sintering nickel powder, impregnating the porous nickel sintered plaque with a nickel salt solution such as an aqueous nickel nitrate solution, and then forming a nickel hydroxide active material in the porous nickel sintered substrate, for example, by immersion in an aqueous alkali solution. The increase of the capacity of such electrodes is limited because the porosity of the plaque is difficult to increase further, so that the amount of the active material incorporated cannot be increased.
An example of the non-sintered type positive electrodes is the electrode disclosed in U.S. Pat. No. 4,251,603 which is produced by filling nickel hydroxide as an active material in the pores of a sponge-like porous plaque with a porosity of more than 95% made of a nickel metal having a multiplicity of cells connected with each other three-dimensionally. This electrode is widely used at present as the positive electrode of an alkaline storage battery having a high capacity.
As to this non-sintered type positive electrode, it has been proposed that spherical nickel particles be filled in the pores of the sponge-like nickel porous plaque in order to increase the capacity. In this case, the pore size of the sponge-like nickel porous plaque is approximately 200-500 .mu.m and spherical nickel particles with a diameter of several micrometers to several ten micrometers are filled in the pores. When such a structure is employed, the charge and discharge reactions of nickel hydroxide particles near the skeleton of the nickel porous plaque proceed smoothly because the particles retain electroconductivity, but those of nickel hydroxide particles apart from the skeleton do not proceed sufficiently.
Therefore, in the non-sintered type positive electrode, an electroconductive agent is used in addition to the nickel hydroxide active material in order to increase the utilization of the nickel hydroxide filled, whereby spherical nickel hydroxide particles are electrically connected to form an electroconductive network. As the electroconductive agent, there are used cobalt compounds (e.g. cobalt hydroxide and cobalt monooxide), metallic cobalt, metallic nickel, etc. Thus, it becomes possible to maintain the electroconductivity in the non-sintered type positive electrode even if the active material is impregnated into the porous plaque at a high density, and hence the capacity can be increased.
U.S. Pat. No. 5,348,822 and the like disclose the improvement of nickel hydroxide itself as an active material for the purpose of increasing the capacity.
Nickel hydroxide which has heretofore generally been known and are used as the active material of the non-sintered type positive electrode is one which is called active nickel hydroxide (.beta.-Ni(OH).sub.2) at discharged state. The average valence of the nickel is about 2.1. This active material becomes .beta.-type nickel oxyhydroxide (.beta.-NiOOH) at charged state. In this case, the average valence of the nickel is said to be around 3.1.
Therefore, when charge and discharge are carried out, the utilization of nickel hydroxide becomes 100% in substantially one-electron reactions. The utilization is the percentage of an actually measured capacity value based on a theoretical capacity value per unit weight 289 mAh/g calculated by assuming the occurrence of the one-electron reactions. Consequently, when said active material is used, the energy density of the positive electrode becomes about 650 mAh/cc.
It was also confirmed that when a nickel/metal hydride storage battery is continuously overcharged at a low temperature, the valence of nickel of the positive electrode is further increased to 3.67.
However, when the average nickel valence exceeds about 3.5, nickel hydroxide becomes .gamma.-type nickel oxyhydroxide (.gamma.-NiOOH). .gamma.-NiOOH is a substance having a diffraction peak on (003) plane at an angle of diffraction 2.theta. of about 12.degree. (.lambda.=1.5405) in X-ray diffraction using CuK.alpha. as a source. Since this substance has cations, anions, water, etc., which are inserted between nickel-nickel metal faces, its crystals are more easily expanded than those of .beta.-NiOOH (density: 4.68 g/cm.sup.3).
In addition, .gamma.-NiOOH (density: 3.79 g/cm.sup.3) becomes .alpha.-3Ni(OH).sub.2.2H.sub.2 O (density: 2.82 g/cm.sup.3) upon discharge. The density change in this case is remarkable, so that the active material repeats its expansion and shrinkage. Therefore, spherical nickel hydroxide particles lose the spherical shape.
Moreover, when .gamma.-NiOOH at charged state is accumulated without discharge, the positive electrode is swollen and absorbs an electrolyte in the battery. Consequently, the volume of the electrolyte held by a spacer is decreased, so that the spacer lacks the electrolyte. Therefore, the internal resistance of the battery is increased to make discharge impossible.
These phenomena have been known since early times also in the case of using a sintered type positive electrode. In particular, a sealed battery is deteriorated in battery characteristics by swelling of the positive electrode.